1. Field of the Invention
The present invention relates to a projection system, and more particularly, to a projection system of the single panel type which is optically efficient and compact.
2. Description of the Related Art
Projection systems are classified into 3-panel projection systems and single-panel projection systems according to the number of light valves for controlling the on/off operation of light emitted from a high-output lamp on a pixel-by-pixel basis and forming a picture. Single-panel projection systems can have a smaller optical system than three-panel projection systems but provide an optical efficiency of ⅓ of that of three-panel projection systems because the red (R), green (G), and blue (B) colors into which white light is separated are used in a sequential method. Hence, attempts to increase the optical efficiency of single-panel projection systems have been made.
Generally, in a single-panel projection system, light radiated from a white light source is separated into R, G, and B colors using a color filter, and the three colors are sequentially sent to a light valve. The light valve appropriately operates according to the sequence of colors received and creates images. As described above, a single-panel optical system sequentially uses colors, so the light efficiency is reduced to ⅓ of the light efficiency of a three-panel optical system. A scrolling method has been proposed to solve this problem. In a color scrolling method, white light is separated into R, G, and B colors, and the three colors are sent to different locations on a light valve. Since an image is produced when all of the R, G, and B colors for each pixel reach the light valve, color bars are moved at a constant speed using a variety of methods.
In a conventional single-panel scrolling projection system, as shown in FIG. 1, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104 and a polarized beam splitter array 105 and is separated into R, G, and B beams by first through fourth dichroic filters 109, 112, 122, and 139. To be more specific, the red beam R and the green beam G, for example, are transmitted by the first dichroic filter 109 and advance along a first light path I1, while the blue beam B is reflected by the first dichroic filter 109 and travels along a second light path I2. The red beam R and the green beam G on the first light path I1 are separated by the second dichroic filter 112. The second dichroic filter 112 transmits the red beam R along the first light path I1 and reflects the green beam G along a third light path I3. The projection system also includes various lenses 107, 117, 120, 131, 137, 140, and 145.
As described above, the light emitted from the light source 100 is separated into the red beam R, the green beam G, and the blue beam B, which are scrolled while passing through corresponding first through third prisms 114, 135, and 142. The first through third prisms 114, 135 and 142 are disposed on the first through third light paths I1, I2, and I3, respectively, and rotate at a uniform speed such that R, G, and B color beams are scrolled. I2 includes a mirror 133. The green beam G and the blue beam B that travel along the second and third light paths I2 and I3, respectively, are transmitted and reflected by the third dichroic filter 139, respectively, and then combined. Finally, the R, G, and B beams are combined by the fourth dichroic filter 122. The combined beam passes through a polarized beam splitter 127 and forms a picture using a light valve 130.
The scrolling of the R, G, and B color bars by the rotation of the first through third prisms 114, 135, and 142 is shown in FIG. 2. Scrolling represents the movement of color bars formed on the surface of the light valve 130 when prisms corresponding to the R, G, and B colors are synchronously rotated.
The light valve 130 processes picture information depending on an on-off signal for each pixel and forms a picture. The formed picture is magnified by a projecting lens (not shown) and is projected onto a screen.
The light source 100 may be, by way of non-limiting example, a xenon lamp, a metal-halide lamp, an UHP lamp, or the like. These lamps are not without disadvantages. They emit many unnecessary infrared rays and ultraviolet rays and generate a great amount of heat, and accordingly necessitate a cooling fan. But, the cooling pan can cause noise. Also, the lamp light source has a narrow color spectrum which reduces the width of color selection, degrades color purity, and cannot be stably used because of its short life. In addition, because the light source 100 emits white light, color filtering units, such as, dichroic filters, are required to separate the white light according to a wavelength. Thus, reducing the size of a projection system is difficult.
Since the conventional single-panel scrolling projection system uses different light paths provided for individual colors, a light path correction lens must be provided for each color, and a component part for re-collecting separated light beams must be provided for each color. Accordingly, an optical system becomes larger, and the yield degrades due to a complicated manufacturing and assembling process. In addition, a large amount of noise is generated due to the driving of three motors for rotating the first through third prisms 114, 135, and 142, and the manufacturing costs of the conventional projection system is higher than a color wheel method adopting only one motor.
In order to produce a color picture using a scrolling technique, color bars as shown in FIG. 2 must be moved at a constant speed. Since the conventional projection system must synchronize a light valve with three prisms in order to achieve scrolling, controlling the synchronization is not easy.